It is known that a latent electrostatic image can be developed with toner particles dispersed in an insulating non-polar liquid. Such dispersed materials are known as liquid toners. A latent electrostatic image may be produced by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. However, other methods are known for forming latent electrostatic images. For example, one of these methods involves providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface.
After the latent electrostatic image has been formed, the image is developed by colored toner particles dispersed in a non-polar liquid. The image may then be transferred to a receiver sheet.
Useful liquid toners comprise a thermoplastic resin and a dispersant non-polar liquid. Generally, a suitable colorant, such as a dye or pigment, is also present. The colored toner particles are dispersed in a non-polar liquid which generally has a high volume resistivity in excess of 10.sup.9 ohm-centimeters, a low dielectric constant (i.e., below 3.0) and a high vapor pressure. Generally, the toner particles are less than 30u average by area size as measured using the Malvern 3600E particle sizer.
Since the formation of proper images depends on the difference of the charge between the liquid developer and the latent electrostatic image to be developed it has been found desirable to add a charge director compound and preferably other adjuvants which increase the magnitude of the charge, e.g., polyhhydroxy compounds, aminoalcohols, polybutylene succinimide compounds, aromatic hydrocarbons, metallic soaps, etc., to the liquid toner comprising the thermoplastic resin, the non-polar liquid and the colorant.
The focus of much of the work in this area has centered around the composition of the resin particles employed in the developer, since the properties of the resins are known to be directly correlated to image quality.
To improve image quality, conventional liquid developers have often been made tacky in order to increase adhesion to the receiver and thus improve transfer efficiency. Generally, tackiness can be achieved in a number of ways, e.g., through the addition of solvents which partially dissolve the resins which make up the resin particles; through the addition of low molecular weight resin fractions; and/or through the control of polymerization of the resin to produce broad molecular weight distributions. For example, U.S. Pat. No. 3,850,829 discloses negative liquid toners containing a tacky organosol means formed by dissolving a high molecular weight resin polymer in an aromatic hydrocarbon solvent and a release agent. Sticky or tacky developers produced in this manner may be disadvantageous, since they may not sufficiently redisperse upon settling. Also, due to their tacky nature, such developers are difficult to clean from photoreceptors. Additionally, cosolvents used in such developers add an undesirable odor to the developer suspension.
Also, as an attempt to improve image quality, resin modification has been heretofore proposed. For example, U.S. Pat. No. 3,993,483 discloses a liquid electrostatic developer for use in developing latent electrostatic images containing at least one member selected from two groups, including a styrene-vinyltoluene copolymer and polyethylene. The developers of this patent incorporate a coloring agent and charge director compounds, such as surfactants. U.S. Pat. No. 3,976,583 discloses electrostatic developer liquids comprising a carrier liquid in which is dispersed a solvent organic liquid, a copolymer of vinyltoluene or styrene with an acrylic acid ester, a copolymer of butadiene with styrene, a coloring agent and water. U.S. Pat. Nos. 4,264,699; 3,997,488; and 4,081,391, all to Tsuboko et al., disclose liquid developers containing resins comprising graft copolymers. These copolymers contain a polar polyester resin compound, a polyethylene wax, and a third copolymer which may be a vinyltoluene-acrylate copolymer.
U.S. Pat. No. 4,794,651 to Landa et al. discloses liquid developers comprising resin particles having fibers or tendrils. Such toners have demonstrated superior image quality in comparison to conventional liquid developers. It is believed that image quality is improved by such toners, since the resin particles are more resistant to breakup during transfer due to the intermingling of the fibers when the particles are concentrated on the photoreceptor.
Liquid toners comprising resin particles having fibers or tendrils were further disclosed and improved upon in U.S. Pat. Nos. 4,760,009; 4,707,429; 4,772,528; and 4,740,444. Specifically, these references disclose improved processes for the production of such resin particles, as well as the dispersion of certain adjuvants in the resin particles.
However, even in light of such modifications, liquid developers heretofore proposed provided a sharply reduced image quality with varying transfer conditions.
To be effective, liquid electrostatic developers must: (1) be attracted to and adhered to a photoreceptor which bears an electrostatic image pattern; and (2) transfer from the photoreceptor to a receiver (generally paper) under the influence of an applied electric field. The transfer from the photoreceptor to a receiver is affected by many external factors, such as temperature, humidity, receiver dielectric constant and surface texture, photoreceptor charge relaxation rate and surface properties, developer conductivity, etc. It is difficult and expensive to precisely control all of these factors and accordingly, it is desirable for developers to transfer uniformly under a wide range of applied fields and conditions. This property can be referred to as transfer latitude. Specifically, transfer latitude refers to the range of applied voltage under which a toner will transfer to a receiver without degradation of image quality. Generally, liquid electrostatic developers having a high mobility and an increased concentrate shear viscosity demonstrate a wide transfer latitude. Conventional developers generally demonstrate a restrictively narrow transfer latitude (i.e., provide adequate images only under a narrow range of applied voltages) which places unnecessarily rigorous demands on the tolerances of the transfer system.
Also, toners with wide transfer latitudes tend to give good transferred images from many different types of photoreceptors without the necessity of being specifically reformulated to suit the individual requirements of each type of photoreceptor. As many types of photoreceptors are known and used in the art, developers with wide transfer latitudes would clearly be advantageous.
Because of their narrow transfer latitudes, conventional developers demonstrate difficulties in providing multiple layer images. For example, one method of providing multiple layer images is to transfer one layer at a time to a receiver sheet without fusing the toner between transfers. A requirement of such a process is that a layer of toner particles must remain on the receiver sheet during all subsequent transfers. However, with conventional developers, the layers frequently become separated from the receiver during subsequent transfer and adhere instead to the photoreceptor, causing a loss of image quality. This phenomenon can be referred to as "back-transfer." As a general rule, "backtransfer" increases as tackiness of the developer increases, and decreases as mobility increases and shear viscosity is optimized.
Also, the surfaces of photoreceptors can be contaminated by trace amounts of impurities in the developer. This contamination can decrease image quality by reducing the developer's ability to adhere to the photoreceptor. Generally, it has been found that developers with a low mobility are more susceptible to the effects of drum contamination.
Therefore, it is an object of the present invention to provide a liquid electrostatic developer for developing latent electrostatic images which improves image quality, even as transfer conditions vary. An additional object of the present invention is to provide a liquid electrostatic developer which provides good multiple layer images without being degraded by backtransfer. A further object of the present invention is to provide toners which avoid the detrimental effects caused by photoreceptor surface contamination. An even further object of the present invention is to provide a liquid developer which transfers well without necessarily being tacky.